A large portion of the chlorine and alkali metal hydroxide production throughout the world is manufactured in diaphragm-type electrolytic cells wherein opposed anode and cathode are separated by a fluid permeable diaphragm, usually of asbestos, defining separate anode and cathode compartments. In a typical operation, saturated brine is fed to the anode compartment wherein chlorine is generated at the anode, the brine percolating through the diaphragm into the cathode compartment wherein sodium hydroxide is produced at a concentration within the range of 11 to 18 percent and "contaminated" with large amounts of sodium chloride. The sodium hydroxide must then be concentrated by evaporation, and the chloride must be removed to provide a commercial product.
Through the years, substitution of a membrane material for the diaphragm has been proposed. These membranes are substantially impervious to hydraulic flow. In operation, an alkali metal chloride solution is again introduced into the anode compartment wherein chloride is liberated. Then, in the case of a cation permselective membrane, alkali metal ions are transported across the membrane into the cathode compartment. The concentration of the relatively pure alkali metal hydroxide produced in the cathode compartment is determined by the amount of water added to this compartment, generally from a source exterior to the cell. While operation of a membrane call has many theoretical advantages, its commercial application to the production, for example, of chlorine and caustic has been hindered owing to the low current efficiencies obtained and the often erratic operating characteristics of the cell.
More recently much improved membranes have been developed to overcome many of the prior problems. The must important such membrane is a thin film of fluorinated copolymer having pendant sulfonyl fluoride groups thereon such as described in U.S. Pat. Nos. 3,041,317; 3,282,875; and 3,624,053 and the like. Such membranes in hydrolyzed form are available from E. I. duPont de Nemours & Company under the trademark NAFION.RTM..
These membranes can be further improved by surface treatments which consist of reacting the sulfonyl fluoride pendant groups with ammonia gas or more preferably with an amine which will yield less polar binding and thereby absorb fewer water molecules by hydrogen bonding such as described in detail in U.S. Pat. No. 4,081,349. The more efficient of these modified membranes are highly cross-linked and become extremely brittle especially in commercial dimension.
To futher improve on these modified membranes, a fabric reinforced material has been laminated to such membranes by the application of heat and pressure. Such treatment, however, has resulted in improvement from a mechanical standpoint but is found lacking in that the heat and pressure required in the fabric bonding operation impairs, if not completely destroys, the effectiveness of the amine modified surface. Methods have now been described, however, which allow the mechanical advantages of a bonded membrane without losing the chemical advantages of the amine modification such as, for example, the method described in U.S. Pat. No. 4,147,844.
In all of these membrane technologies, however, there continues to be the normal degradation of the membrane over time during its operating life. The amount of time before these membrane cells become economically unacceptable has, in the past, been rather short, and therefore costly downtime and replacement costs are incurred in using membrane cells in the chlor/alkali manufacturing process.